Conventionally, an alarm device has been proposed having an oscillation section which comprises an oscillator having an oscillation coil, a detector for detecting the frequency or amplitude, a comparison circuit for emitting output pulses by comparing the output of the detector with a standard signal, and an integration circuit for integrating the output pulses of the comparison circuit. All of this apparatus was positioned on a vehicle body in the vicinity of a wheel. However, the oscillation section of the conventional alarming device was relatively complicated and bulky and settled at a position in the neighbourhood of the rotating vehicle wheel so that the oscillation section was readily damaged. Moreover, the oscillation section was apt to receive such effects as radiant heat radiated from the vehicle wheel, conductive heat conducted through a bracket supporting the oscillation section, and heat stagnated in a tire, with the result that the temperature of electric parts or elements used in the oscillation section was heightened to exceed an allowable limit of 70-80.degree. C. It was therefore impossible to signal a driver with a high degree of reliability. In order to position the oscillation coil at all times in opposing relation with a resonance coil attached to the vehicle wheel in such a tire inner pressure drop alarming device, the oscillation coil was required to be settled below the springs of the vehicle which receives an extremely large vibrations so that the durability and reliability of the oscillation section decreased with time, resulting in obstacles for enhancing the reliability of the tire inner pressure drop alarming device. Furthermore, it was a common practice to position the resonance coil outside of and immediately below a rim flange so as to avoid damages caused by collision with many obstacles on the road. If the resonance coil has an entirely circular cross-section under the state that the resonance coil is assembled outside of and immediately below the rim flange, a maximum entire circle diameter is necessarily restricted and a radial electric magnetic wave from the oscillation coil opposing the resonance coil was not effectively received by the resonance coil, which makes it impossible to emit an alarm with a high degree of reliability.